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I bet you when the tech said it was overcharged by 1 1b, it wasnt an exact figure, that 1 lb if it was, was just to get his readings correct. If the system was off for alittle while, the ducts would be hot giving a higher temp loose from the A/H to the air outlets. There is going to be some increase in temp when the ducts are in a hot location, from the time the air leaves the A/H to the time it exits the air outlets (7 deg is alot). The tech that came out was mostlikly from the installing contractor and is not going to say anything too negitive if the system was undersized. IMO, unless you have a very well insulated home and is some kinda energy eff home, and is sitting under a blanket, I usually (depends) use 400-450 sq ft per ton, sometimes 350 (they used about 700 Sq ft per ton) I am not a big resi guy, and would not be able to to a load calculation for you on the computer. I see your not going to get too much sleep over this. Bring in another HVAC company to help you out. The system may be able, w/ some very fine tweeking, to do alttle better, I dont think so though, it sounds some what small.

what was....

....the superheat or subcooling? Ive found noncondensibles in new homes many times. The builders will let their hack installers do start ups and they dont have a clue as to what they are doing. I had one tell me that if he has 70 suction pressure and 250 liquid pressure then he calls it good. Another thing I can tell you about new homes is that the attics are ALWAYS under ventilated. Im amazed at how these so called builders get by with this crap. Oh, and by the way, I bet they used an Allstyle plenum coil and I KNOW they didnt put a drier on the liquid line. Sorry if I sound so negative but I really hate home builders. I equate them with lawyers.

Can't comment on whether your system is undersized or not as have no way to see the house from here. Good attic ventilation is worth about 1/2 ton of A/C so if you think you need a bigger unit why not try putting a heat activated roof vent fan in the roof. If the ductwork is not adequatly insulated dropping the temperature in the attic by means of good ventilation will certainly go a long way towards lowering the temperature at the vents. I generally don't like to see temps in excess of 60 degrees at the registers. 54 or 55 would be a far better cooling temp. If things are set up correctly the system should be sized to pull the temp down to 75 degrees on a 90+ degree day. What is temperature during the hottest part of the day in your attic?

I think you said it was 53 % humidity - that can play a big role. Before you can cool the air you need to take the moiture out of the air...speculation...2.5 is a little small - I installed in my own house (1700 sq feet) a 3 ton - 600 sq feet per ton is what I go by - but in more humide areas I would go a little larger to handle the extra load. Do you have a programable stat - if so give the unit somemore run time to help it get an early start on the hot day.

All duct is R-6 and insulation in attic is R-22 with and adv of 14 inches loose fill

What are your thoughts of increasing preformance so the a/c doesn't have to run all day?
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1.)Increase R-value of return air and other ducts?
- it would be a pain for me the diy to do.

2.) Put in powered ventilation either gable or roof mount
- I am afraid if I put in powered roof mount it will just suck air in from the other vents in the roof. A gable vent at the garage might be the best bet and easiest

3.) Complain to the home builder that the attic isn't vented properly
- I bet this will get me no where

4.) Complain that the a/c is undersized

5.) Check with the neighbors and see if they are having the same problem